The present invention relates to communications satellites and, more particularly, to communications satellites in geostationary orbit.
The most desirable orbit for communications satellites is the geostationary orbit 22,300 miles above the earth's surface over the equator, and more than 60 satellites have been launched into that orbit at the present time. The reason for the popularity of the geostationary orbit is that a satellite placed there remains fixed above a selected place on the earth's surface.
Some portions of the geostationary orbit tend to be crowded at certain longitudes because satellites at those locations are in view of portions of the earth's surface involving heavy communications traffic. Such desirable positions are, for example, those over the Atlantic Ocean, the Pacific Ocean, the Indian Ocean and the longitudes that pass through the American Continent. The result is that the most useful geostationary orbit locations are becoming saturated.
There is also a problem with radio frequency congestion. Only certain portions of the radio spectrum are allocated for communication satellite use. When several geostationary satellites operate in the same frequency band, discrimination between the signals from adjacent satellites is achieved primarily by the angular separation between the earth station antennas, thus avoiding mutual interference. Approximately 40.degree. of the geostationary orbit is desirable for North American communications and will permit the use of approximately ten geostationary satellites using the same frequency band and separated in orbit by approximately 4.degree. of orbital arc. The presently orbiting satellites have almost exhausted the available North American orbital slots at C-band (4/6 GHz). However, other satellites can be placed in the same orbital segment at other frequency bands, such as Ku band (11/14 GHz) and Ka band (20/30 GHz).
As is generally well-known, the frequency and orbital position assignments are allocated by an international body, the World Administrative Radio Conference (WARC) of the International Telecommunications Union (ITU). Frequency and orbital congestion have led to problems of allocation among nation. Equatorial nations now wish to claim rights to the portions of the geostationary orbit above their countries, and the underdeveloped countries wish to obtain a portion of the equatorial orbit and the frequency spectrum before it has all been allocated or assigned to the developed countries. Hence, it appears that the situation will become worse in time.
One of the presently proposed solutions to these problems is to place a large structure in space which would provide the equivalent performance or better of a large number of individual satellites. Such a structure has been referred to variously as a space station, a space platform, a switchboard in the sky, an orbital antenna farm (OAF) and a geostationary communications platform. Descriptions of such structures are given in the following articles, amoung others: Edelson, B. I., and Morgan, W. L., "Orbital Antenna Farms," Astronautics and Aeronautics, September 1977, pp. 20-27; Fordyce, S. W. and Jaffe, L., "Future Communications Concepts: Switchboard-in-the-Sky," Satellite Communications, February 1978, pp. 22-26, and Mar. 1978, pp. 22-27; and Morgan, W. L., "Space Stations," Satellite Communications, April 1978, pp. 32-39.
In general, such a large space station would be assembled and fastened together into an integral structure in orbit from components ferried into space by a vehicle such as the Space Shuttle. The space station would have a plurality of different transmitters, receivers, and antennas mounted to a common support structure and sharing a common power supply, common support and station keeping apparatus, and the like. In addition, the various transmitters and receivers would be connected by means of waveguides and cables with an on-board signal processor which would provide functions such as switching, demodulation and remodulation, for example. On subsequent visits of the Space Shuttle, additional equipment could be added to increase communications capacity or to provide additional services, failed equipment could be repaired or replaced, and obsolete equipment could be replaced with newer equipment. By the use of large, high gain antennas and special antenna feeds, multiple narrow spot beams could be formed, permitting reuse of the same frequency spectrum without excessive mutual interference.
Clearly, such large space stations are desirable because they would provide all of the communications services presently available from a multiplicity of satellites in orbit, and yet each space station would occupy only a single orbital slot, thereby relieving orbital congestion. The use of large aperture antennas would provide multiple narrow spot beams, and cross-linking by on-board signal processors would permit receiving a signal at one frequency band and retransmitting it at another frequency band, This would alleviate the frequency congestion problem and provide many more communications channels. The system would be repairable, and would be flexible as to adding additional capacity or other types of service.
However, the problem with putting large space stations in orbit is that the presently available technology is not yet adequate to permit the immediate launch of the types of space stations that are presently contemplated. A great deal of design, development and experimentation must be done, particularly in the areas of space transportation systems and the construction or assembly of large objects in space. These developments depend on large investments of funds that are yet to be committed.